Conventional wafer bonding techniques are generally categorized as fusion, eutectic or hybrid bonds. Fusion bonding is the adhesion of two substrates placed in direct contact with each other. At room temperature, the substrates bond to one another following a chemical or plasma activation step. Eutectic bonding is the adhesion of two substrates, having a eutectic metal layer positioned between them, where the metal transforms directly from a solid to liquid state, or vice versa, at a specific composition and temperature. A hybrid bond, as the name implies, is simply a combination of the fusion and eutectic bonds.
A variety of drawbacks accompany conventional boding techniques. For example, the fusion bond provides no electrical contacts between the substrates. As such, further costly processing of the wafer is necessary to achieve interconnection of the metal layers. Eutectic bonding requires high compressive forces and temperatures, resulting in poor accuracy and potentially damaging mechanical stresses. Lastly, loading associated with polishing techniques (e.g., planarization), dishing and oxidation are all concerns in hybrid bonding.